/*
OC Volume - Java Speech Recognition Engine
Copyright (c) 2002-2004, OrangeCow organization
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 */
package org.ioe.tprsa.audio.feature;

/**
 * last updated on June 15, 2002<br>
 * <b>description:</b> FFT class for real signals. Upon entry, N contains the
 * numbers of points in the DFT, real[] and imaginary[] contain the real and
 * imaginary parts of the input. Upon return, real[] and imaginary[] contain the
 * DFT output. All signals run from 0 to N - 1<br>
 * <b>input:</b> speech signal<br>
 * <b>output:</b> real and imaginary part of DFT output
 * 
 * @author Danny Su
 */
public class FFT {

        /**
         * number of points
         */
        protected int numPoints;
        /**
         * real part
         */
        public float real[];
        /**
         * imaginary part
         */
        public float imag[];

        /**
         * performs Fast Fourier Transformation<br>
         * 
         * @param signal
         */
        public void computeFFT(float signal[]) {
                numPoints = signal.length;
                // initialize real & imag array
                real = new float[numPoints];
                imag = new float[numPoints];
                // move the N point signal into the real part of the complex DFT's time
                // domain
                real = signal;
                // set all of the samples in the imaginary part to zero
                for (int i = 0; i < imag.length; i++) {
                        imag[i] = 0;
                }
                // perform FFT using the real & imag array
                FFT();
        }

        /**
         * performs Fast Fourier Transformation<br>
         */
        private void FFT() {
                if (numPoints == 1) { return; }
                final double pi = Math.PI;
                final int numStages = (int) (Math.log(numPoints) / Math.log(2));
                int halfNumPoints = numPoints >> 1;
                int j = halfNumPoints;
                // FFT time domain decomposition carried out by "bit reversal sorting"
                // algorithm
                int k = 0;
                for (int i = 1; i < numPoints - 2; i++) {
                        if (i < j) {
                                // swap
                                float tempReal = real[j];
                                float tempImag = imag[j];
                                real[j] = real[i];
                                imag[j] = imag[i];
                                real[i] = tempReal;
                                imag[i] = tempImag;
                        }
                        k = halfNumPoints;
                        while (k <= j) {
                                j -= k;
                                k >>= 1;
                        }
                        j += k;
                }

                // loop for each stage
                for (int stage = 1; stage <= numStages; stage++) {
                        int LE = 1;
                        for (int i = 0; i < stage; i++) {
                                LE <<= 1;
                        }
                        int LE2 = LE >> 1;
                        double UR = 1;
                        double UI = 0;
                        // calculate sine & cosine values
                        double SR = Math.cos(pi / LE2);
                        double SI = -Math.sin(pi / LE2);
                        // loop for each sub DFT
                        for (int subDFT = 1; subDFT <= LE2; subDFT++) {
                                // loop for each butterfly
                                for (int butterfly = subDFT - 1; butterfly <= numPoints - 1; butterfly += LE) {
                                        int ip = butterfly + LE2;
                                        // butterfly calculation
                                        float tempReal = (float) (real[ip] * UR - imag[ip] * UI);
                                        float tempImag = (float) (real[ip] * UI + imag[ip] * UR);
                                        real[ip] = real[butterfly] - tempReal;
                                        imag[ip] = imag[butterfly] - tempImag;
                                        real[butterfly] += tempReal;
                                        imag[butterfly] += tempImag;
                                }

                                double tempUR = UR;
                                UR = tempUR * SR - UI * SI;
                                UI = tempUR * SI + UI * SR;
                        }
                }
        }
}